Turbofan engines typically have a case assembly with a fan case, an intermediate case, a compressor case, a gas generator case, a turbine case and a turbine exhaust case about a centreline. The hot section of the engine, including the gas generator case, the turbine case and turbine exhaust case, are typically made of steel or nickel alloys. The cooler sections such as the intermediate case and the compressor case may be made of lighter materials such as aluminium or magnesium. However, steel is conventionally used for the fan case because of its strength.
In the intermediate case, there is a compressor air passage, and a bypass air passage, defined by an annular splitter. The splitter extends forward of radial struts integrated between the intermediate case and the fan case. The intermediate case with the splitter and the struts is generally integrally cast and then machined. The parts are of uniform material thickness resulting in above limit stresses in certain locations.
Low cycle fatigue is a persistent problem which can reduce the useful life of various structural components due to cycling between idling and operating conditions and thus subjected to stresses generated by thermal expansion effects and mechanical loads. Due to the high loads experienced on intermediate case designs, low cycle fatigue locations are especially noted on all struts. Increasing the mass to compensate increases the thermal stresses, while decreasing mass increases the g loading and thrust contribution.
Improvement in case design is desired.